The present invention relates to an objective lens which is installed in an optical information recording/reproducing apparatus for recording information to and/or reproducing information from an optical disc complying with a predetermined standard.
There exist various standards of optical discs (CD, DVD, etc.) differing in recording density, protective layer thickness, etc. Meanwhile, new-standard optical discs (e.g., BD (Blu-ray Disc), having still higher recording density than DVD, are being brought into practical use in recent years to realize still higher information storage capacity.
Since such a high recoding density optical disc has a recording density higher than that of CD or DVD, it is necessary to form a smaller beam spot for information recording or information reproducing for the high recording density optical disc. In other words, a higher NA (Numerical Aperture) is required for information recording or information reproducing for the high recording density optical disc. For this reason, in general, a glass lens having a relatively high refractive index is employed as an objective lens for the optical information recording/reproducing apparatus for the high recording density optical disc. An example of such a glass lens for an optical pick-up is disclosed in Japanese Patent Provisional Publication No. 2005-156719A.
However, a glass lens has a drawback that since a glass lens is heavy in weight, the glass lens places a burden on operation of an actuator for driving the lens when the actuator moves the lens in a focusing direction or in a tracking direction. Therefore, a designer might consider forming a lens with resin in place of glass in order to reduce weight of the lens.
Japanese Patent Provisional Publication No. 2003-5032A (hereafter, referred to as JP 2003-5032A) discloses an objective lens made of resin for the high recording density optical disc. JP 2003-5032A discloses that by reducing weight of the objective lens by forming the lens with resin, the burden on the actuator can be reduced, and it becomes possible to enable the actuator to perform the precise position control.